1. Field of the Invention
This invention relates generally to telecommunications, and more particularly, to wireless communications.
2. Description of the Related Art
Mobile communication systems are increasingly being utilized to transfer information, such as data, voice, text or video, among communication devices on a wireless network. In fact, huge demands for a wide range of data services in a mobile environment are imposed and a provision of a high Quality of Service (QoS) similar to that provided by fixed networks is desired. To this end, a number of standards for network technologies and communication protocols have been proposed or suggested, rendering a variety of services to users. For example, a third generation partnership project (3GPP) standardization has introduced an Internet Protocol (IP) Multimedia Subsystem (IMS) to support new services. The IMS provides IP based multimedia services to users through a network based on the Universal Mobile Telecommunications System (UMTS) standard. Moreover, an IMS service plays a role for the QoS in a radio access network, such as a Universal Mobile Telecommunications System Terrestrial Radio Access Network (UTRAN). Radio bearers, i.e., services provided for the transfer of user data between user equipment (UE) and UTRAN are designed in a way that is flexible enough to cope with the introduction of future IMS services such as real time multimedia.
Typically, mobile communication systems include a plurality of cells, each of transmitting signals to and receiving signals from mobile stations within its coverage or service area. For example, a coverage or service area of a wireless communication network, such as a digital cellular network is generally partitioned into connected service domains known as the cells, where cellular phone users can communicate, via radio frequency (RF) links, with a base station serving the cell. The cells may be further partitioned into segments, typically three to a cell. A base station may be assigned a plurality of channels within a frequency spectrum over which it can communicate with a mobile station. A mobile station within range of the base station may communicate with the base station using these channels. In general, the channels used by a base station are separated from one another in some manner so that signals on any channel do not substantially interfere with signals on another channel used by that base station or other adjoining base stations. Therefore, for mobile communication systems in which areas served by a wireless network are divided into cells, a way is desired for dynamically allocating available system channels to wireless devices, such as mobile stations requiring service.
The UMTS standard allows the transmission of data (user or control) in two different states, namely a cell dedicated channel (CELL_DCH) state and a cell forward access channel (CELL_FACH) state. Both states can be characterized by their channel usage and have a specific behavior, which makes them suitable for carrying different types of traffic. In the CELL_DCH state, a dedicated connection exists over a dedicated channel (DCH). Due to the application of a closed loop power control and soft handover (SHO), the required transmission power is minimized. On the other hand, for establishing, reconfiguration and release of a DCH, radio resource control (RRC) procedures are used. These RRC procedures cause overhead and radio network controller (RNC) signaling load, which may become significant, especially for short packet transmissions. Hence, the CELL_DCH state is well suited for the transport of large packets, such as packet downloads or conversational traffic.
In the CELL_FACH state, data is transmitted over a random access channel (RACH) in an uplink and a shared forward access channel (FACH) in a downlink. Due to an open loop power control on the FACH, the transmit power requirement is larger than on the DCH and the reliability at the cell border may be less predicable than on the DCH. In addition, the FACH will have a low data rate, typically of the order of 32 kbps to 64 kbps in order to keep the transmit power and interference to an acceptable level, this means that the channel may become congested when multiple users are accessing it. On the other hand, packet handling is performed in a medium access control (MAC) layer and, hence, no overhead due to dedicated signaling procedures is added. Therefore, the CELL_FACH state is a preferred state to transport short and infrequent data, such as background traffic and signaling.
From the characteristic above it may be concluded that RRC signaling, which is exchanged between a UE and a core network (CN), may be best carried over the CELL_FACH due to its infrequent and short nature. However, because of the lack of the closed loop power control, the CELL_FACH is more vulnerable to channel errors, especially in bad environmental situations, such as near an edge of a cell. This effect causes additional delay of the transmission. Furthermore, because a simple scheduling is used in the CELL_FACH, additional delays may occur. For some services, this delay may be unacceptable even for the RRC signaling.
Moreover, the current UMTS standard defines only the UE states and the procedures necessary to transition between these states. However, no method has been specified to decide which state to use under which scenario. Therefore, it is up to each UTRAN vendor to find a suitable solution for this decisional problem in each implementation of the CELL_FACH state for efficient packet data transmission. There are two scenarios for a decision to move into a different UE state. First, for an already existing packet switched (PS) service, the traffic demand might increase/decrease. In that scenario, if certain thresholds are exceeded or lowered, it is beneficial to switch from the CELL_FACH state to the CELL_DCH state and vice versa. This scenario is covered by the vendor specific algorithms. The second scenario occurs for the establishment of a signaling connection, leading to a question of which UE state is best suited for the signaling connection. In that case, the information of the service is only used to decide if a connection should be established in the CELL_DCH state or in the CELL_FACH state. For example, if it is decided to establish a connection in the CELL_FACH state, in worse environment conditions, such as the cell edge, the connection may experience a bad quality because of the current radio frequency (RF) environment situation. In the extreme case, this may lead to a significant delay and even dropped connections over the FACH, if some timers expire.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.